This shiftable transmission is especially necessary in machines, such as wheel loaders or high-lift trucks since in these vehicles low driving speeds are required when picking up loads, however, the operating hydraulic system requires maximum power in this state. For this state, it is necessary to maintain the driving motor at the maximum possible rotational speed in order to drive the hydraulic pump connected thereto at maximum rotational speed and to operate the friction clutch in a slipping manner, in order to reduce the driving speed. For this, the actuating force of the friction clutch can be lowered upon actuation of the inch pedal wherein, in the case of a minimal actuation path of the inch pedal, the actuating force of the friction clutch is reduced only slightly; the actuating force of the friction clutch is then further reduced during the continued actuation path of the inch pedal and beyond a defined actuation path of the inch pedal the service brake of the vehicle is additionally activated to further lower the driving speed. The friction clutch is exposed to the risk of damage in the slipping state, especially when the frictional power exceeds a defined value. This can occur due to incorrect adjustment of the inch pedal and hence erroneous target values.
WO 00/22323 discloses a reversing gearbox that can be shifted under load in which the pressure in the friction clutch is lowered in such a way that by actuating a hydraulic inch valve said clutch changes into a defined slippage state. Especially hydraulic inch valves are very complex to adjust which, in conjunction with the hysteresis of said hydraulic valve, leads to erroneous target values, and damage of the friction clutch.
DE 198 30 950 A1 discloses a method and a device for actuating a motor vehicle clutch device in which a pedal position is used to generate appropriate voltage via a rotational angle sensor on the clutch pedal or on a separate inch pedal with said voltage being processed in an electric control unit. By way of an inch pedal line, this voltage is converted into the inch effect, which is between 0% and 100%. Thus it is possible to cause the desired slippage for the reversing clutch. In order to protect the clutch from damage, the speed differential of the clutch device is modulated by varying the gear ratio in the inch operation. However, this is only possible with continuously variable gear units and cannot be employed in powershift transmissions with fixed gear ratio steps.
It is the object of the present invention to create a mobile vehicle comprising a shiftable transmission with fixed gear ratio steps and a friction clutch in which the friction clutch is brought into the slipping state in a targeted fashion and in which the friction clutch is protected from damage.